An organic EL display using an organic EL (a spontaneous light emitting type light emitting element) for a display panel has reached a practical level. Since the organic EL display has excellent features which cannot be obtained by a liquid crystal display, for example, spontaneous light emission, high-speed response and a wide angle of view, it has been widely expected to be a flat panel display which can produce clear characters, and graphic images, and a dynamic image display. The organic EL display can be classified into a passive matrix type (PM type) and an active matrix type (AM type) depending on driving method.
It is said that the PM type is provided with a driving circuit on the outside of an organic EL panel and the structure of the organic EL panel itself can be therefore simplified and cost can be reduced. At present, the PM type organic EL panel is manufactured as a product and is used for a vehicle or a mobile telephone. The organic EL is a current driving element. In order to eliminate variation in luminance of the organic EL panel, therefore, it is necessary to cause a current flowing to each light emitting pixel to achieve an equal magnitude. However, it is hard to have an equal current and to reduce power consumption due to the following problems (1) to (3).
(1) In order to cause luminance of all pixels to be uniform, the current flowing to each of the pixels is to be equal. For this reason, it is necessary to cause one of the positive and negative electrodes of each pixel to act as a constant current source. In order to operate the electrode as the constant current source, however, the driving voltage of a matrix electrode on the other side is to be increased such that a voltage drop caused by the resistance component of a bus line has no influence. Consequently, a power consumption is increased. In the case in which a driving voltage cannot be increased sufficiently, a voltage drop corresponding to a bus line length to reach each pixel influences a current amount for light emission. More specifically, the constant current source is not obtained so that a variation in luminance is caused.
(2) In order to obtain a predetermined surface luminance, the PM type needs to emit a light with an N-fold instantaneous luminance if the number of scanning lines of the display panel is N. Since a current flowing to the pixel is usually proportional to a light emission luminance, the current to flow becomes N-fold. Since the organic EL has such a feature that a light emission efficiency is reduced if a current to flow is increased, however, an N-fold pixel current or more is required for obtaining the predetermined surface luminance. Thus, the power consumption is increased if the number N of the scanning lines is increased. This problem increasingly promotes the problem (1).
(3) Since the organic EL panel has a surface structure, a capacitive load is connected to each element in parallel as an equivalent circuit. When a pixel current is increased or the number of pixels is increased so that a repetitive frequency is increased, the magnitude of a charging and discharging current to flow to the capacitive load is made great so that a power consumption is increased. Due to the problem (2), the power consumption of the capacitive load is considerably increased in the PM type.
Due to the above problem, the PM type which is currently manufactured as a product has a screen size of several inches or less and a pixel number of 10,000-pixel level.
In the AM type organic EL panel, the above problems can be alleviated.
In the above problem (1), the AM type has a TFT driving circuit provided in each pixel. Therefore, it is not necessary to cause a large current to flow instantaneously. As a result, a voltage drop caused by a bus line in the above problem A is decreased and an applied voltage can be reduced. Consequently, the power consumption can be reduced more considerably than that of the PM type. Since the applied voltage can be reduced, a slightly high applied voltage is simply set so that a voltage drop corresponding to a bus line length to each pixel does not influence on a pixel current amount. Consequently, an uniform luminance can be obtained.
In the above problem (2), the AM type has a TFT driving circuit provided in each pixel. Therefore, it is sufficient that a small pixel current always flows irrespective of the number N of the scanning lines. Therefore, there can be avoided increase of power consumption due to reduction in light emission efficiency with an increase in a pixel current. In the problem (3), since the AM type has the TFT driving circuit provided in each pixel, it is sufficient that a small pixel current flows irrespective of the number N of the scanning lines. Therefore, a charging and discharging current flowing to the capacitive load can be reduced. Consequently, the power consumption can be reduced. Thus, the AM type organic EL panel can reduce a variation in luminance and a power consumption.
However, the AM type has the following great drawback. More specifically, it is hard to fabricate a driving element having a uniform characteristic over the whole organic EL panel area. As a result, a current value flowing to each pixel is different so that a luminance is varied.
FIG. 7 shows a pixel driving circuit for causing a pixel in a conventional AM type organic EL panel to emit a light which has been described in Japanese Patent Publication No. 2784615, for example.
An operation of the pixel driving circuit will be described below with reference to FIG. 7.
A first transistor 53 is, for example, a FET constituted by an N channel type and is operated as a switching element. A second transistor 55 is, for example, an FET constituted by a P channel and is operated as a driving element. A capacitor 54 is a capacitive load connected to the drain terminal of the first transistor 53. An organic EL element 56 is connected to the drain terminal of the second transistor 54. The drain terminal of the first transistor 53 is connected to the gate terminal of the second transistor 55. A scanning signal is applied from a first vertical scanning line 51 to the gate terminal of the first transistor 53. An image signal is applied from a first horizontal scanning line 52 to the source terminal. Reference numeral 57 denotes a power source line.
Next, light emitting mode is explained. First of all, a scanning signal is applied to the gate terminal of the first transistor 53. At this time, when an image signal is applied at a predetermined voltage to the source terminal of the first transistor 53, the capacitor 54 connected to the drain terminal of the first transistor 53 is held to have a voltage level V1 corresponding to the magnitude of an image signal. If the magnitude of the voltage level V1 held to have the gate voltage of the second transistor 55 is enough for causing a drain current to flow, a current corresponding to the magnitude of the voltage level V1 flows to the drain of the second transistor 55. The drain current becomes a light emitting current for the organic EL element 56. The luminance is proportional to the magnitude of the light emitting current.
FIG. 8 is a characteristic chart for explaining the generation of a luminance variation in a pixel when a light emission is carried out by such an operation. The characteristic chart shows the relationship between a gate-source voltage and a drain current of the second transistor 55. In the case in which the first transistor 53 and the second transistor 55 are formed of the low temperature polysilicon, it is hard to obtain an FET having the same characteristic over the whole display panel area in respect of manufacturing method of a low temperature polysilicon. For example, each of the first transistor 53 and the second transistor 55 has a variation in characteristic shown in FIG. 13. When the voltage level V1 is applied to the second transistor 55 having such a characteristic, the magnitude of the drain current is varied in a range of Ia to Ib. Since the organic EL emits a light with a luminance which is proportional to the magnitude of the current, the characteristic of the second transistor 55 represents a variation in light emission luminance. In particular, the variation in characteristic shown in FIG. 8 cannot prevent the generation of the luminance variation in a method of modulating a luminance in an analog amount, that is, a method of controlling a light emission luminance with the magnitude of the voltage level V1.
In a digital luminance control method for controlling a luminance at a level in which the voltage level V1 shown in FIG. 9 always has a constant value, therefore, a level in which a current is saturated is used. Consequently, it is possible to prevent a luminance variation generated in an analog luminance control method. In the case of a characteristic having the relationship between the gate-source voltage and the drain current of the second transistor 55 shown in FIG. 10, however, a saturation current is not equal. Also in the digital luminance control method, therefore, a luminance variation is generated. Thus, it is difficult to prevent the luminance variation by the characteristic variation of a driving element in the conventional driving circuit. FIG. 11 shows a pixel driving circuit described in “Active Matrix OELD Displays with Po—SiTFT. The 10th International Workshop on Inorganic & OEL. P. 347 to P. 356” as a conventional example in which the characteristic variation of the driving element is improved. In this conventional example, the second transistors 55A, 55B to be driving elements are connected in parallel with each other to average the characteristic variation.
Moreover, there has been proposed a circuit for automatically correcting a characteristic variation in a driving element. FIG. 11 shows an automatic correcting circuit for a driving element characteristic variation provided in a pixel which has been illustrated in {R. Dawson. et al. : Proc. of SID' 99 (1999) P.438}. In the present circuit, two transistors are used in addition to a first transistor and a second transistor, thereby correcting the characteristic variation in the driving element.
With reference to FIG. 12, the operation of the present circuit will be described. First of all, a first vertical scanning line 51 is activated so that a first transistor 53 is conducted and a signal for sufficiently conducting a second transistor 55 is input from a first horizontal scanning line 52 through the first transistor 53 and an auxiliary capacitor 553. At this time, a transistor 555 for organic EL element connection which is to be controlled by a vertical scanning line 552 for organic EL element connection is conducted and a current flows to an organic EL element 56 by a current sent from a power line 57. Next, when the vertical scanning line 552 for organic EL element connection is deactivated and a vertical scanning line 551 for correction is activated, the current of the organic EL element 56 is stopped, while the closed circuit of a capacitor 54, the second transistor 55 and a transistor 554 for correction is formed and the voltage of the capacitor 54, that is, the gate-source voltage of the second transistor 55 is gradually dropped. When the voltage reaches the threshold voltage of the second transistor 55, the second transistor 55 becomes non-conductive so that the closed circuit is opened. Accordingly, an electric potential corresponding to the threshold voltage is stored in the capacitor 54. Next, the transistor 554 for correction is non-conducted by the vertical scanning line 551 for correction to activate the vertical scanning line 552 for organic EL element connection so that the transistor 555 for organic EL element connection is brought into a conduction state. Then, a data signal corresponding to a luminance required for the organic EL element 56 is sent by the first horizontal scanning line 52. Consequently, a specified luminance can be realized. In the present circuit, the threshold voltage of the second transistor 55 is stored in the capacitor 54 in a pixel, thereby correcting a variation in a threshold to reduce a variation in a luminance.
The conventional spontaneous light emitting type display device is constituted as described above. Therefore, there is a problem that a voltage to be applied to the organic EL element is varied in the case in which the threshold voltage of a transistor to be a driving element is varied, and a luminance is varied in each pixel in the case in which display is carried out in the same gradation.
On the other hand, the variation in the threshold voltage of the transistor is canceled in order to suppress the variation in the luminance. As in the structure shown in FIG. 12, therefore, in a spontaneous light emitting type display device using four transistors in one pixel, a variation in the threshold of the transistor can be suppressed. However, an organic EL element has a luminance-element applied voltage characteristic shown in FIG. 13, for example, characteristics A, B and C depending on a variation in a light emission threshold voltage. When the same voltage Vs is applied to carry out the display in the same gradation, the luminance of each pixel generates a variation in values indicated as Bo, Ba and Bb depending on a difference in the light emission threshold voltage. Consequently, it is impossible to suppress the variation in the luminance caused by a variation in the characteristic of the organic EL element itself.